Bolt Torque Calculator (ISO Metric)
Bolt torque calculator. Compute installation torque from ISO grade, metric size, and lubrication condition. T = K·F·d. Per Shigley + standard fastener engineering.
Bolt Torque
| Nut factor K | — |
| Bolt nominal diameter d | — |
| Stress area A_s | — |
| Proof load F_p | — |
| Target preload F (75% proof) | — |
How to use the bolt torque calculator
Pick bolt size + property class
ISO metric: M6 to M36. Property class on the bolt head: 8.8 standard structural, 10.9 high-strength, 12.9 premium aircraft/automotive. The first number × 100 = tensile strength (MPa). Second number × 10 = yield strength as % of tensile.
Choose lubrication
K = 0.10 lubricated: with oil, anti-seize, lubricated threads. K = 0.20 dry/as-received: typical industrial default. K = 0.30 plated/cadmium: zinc-plated or cadmium-plated bolts, higher friction. The same preload requires different torque depending on K.
Set preload target
Industry standard: 75% of proof load. Some critical applications use 90% (preloaded joints) or 50% (vibration-sensitive). Higher preload = better fatigue resistance + joint clamping, but risks yielding the bolt.
Read installation torque
Apply this torque with a calibrated torque wrench. Both N·m and ft·lb shown. Critical: torque accuracy is typically ±15-30% even with calibrated wrenches due to friction variation. For precision applications, use angle-controlled tightening or ultrasonic preload measurement.
Cross-check with manufacturer specs
For safety-critical applications (engine head bolts, structural connections, pressure vessels), always use manufacturer-published torque specs over generic calculations. ISO + ANSI tables in standards documents. Engineering judgment: include thread engagement, joint stiffness, gasket compression in real designs.
Bolt torque — converting twist into clamping force
Bolt torque calculation translates the tightening torque applied at a bolt head into the clamping force ("preload") that compresses the joint. The standard simplified formula is T = K × F × d: torque (T) equals nut factor (K) times preload force (F) times bolt nominal diameter (d). The nut factor K captures all friction effects — thread friction + bearing surface friction under the bolt head/nut. Typical K values: 0.10 well-lubricated, 0.15 lightly oiled, 0.20 dry/as-received, 0.30 plated. The exact K depends on surface finish, thread quality, lubricant, temperature — making torque-based control inherently imprecise (±15-30% scatter typical).
Why preload matters more than torque
The engineering goal is achieving the right preload force (clamping force) — not specifically a torque value. Preload provides: (1) joint clamping that prevents separation under external load, (2) fatigue resistance — preloaded bolts see less stress fluctuation, (3) friction grip for shear loads. Torque is just an indirect way to estimate preload. More precise methods exist: ultrasonic preload measurement (measures bolt elongation directly), angle-controlled tightening (rotate by a specific angle after snug), load-indicating washers (DTI washers that compress visibly). For critical applications (aircraft, nuclear, large structural), torque alone is insufficient.
Torque is the easiest way to install a bolt. Preload is the engineering goal. The gap between them — friction scatter, lubrication variation, thread quality — is why critical applications use angle control or direct preload measurement.
ISO property classes + proof load
ISO 898-1 defines metric bolt property classes by the format X.Y where the first number × 100 = tensile strength (MPa) and the second × 10 = yield as % of tensile. Class 8.8: 800 MPa tensile, 640 MPa yield. Class 10.9: 1000/900. Class 12.9: 1200/1080. Proof load is the maximum permissible elastic load — typically 90-95% of yield. Bolts are designed to operate below proof load under maximum service conditions. The 75%-of-proof-load preload target leaves margin for: external load addition, fatigue stress, thermal expansion stress, retightening over the bolt life.
ASEAN fastener context
Metric ISO bolts are universal in ASEAN industries. Singapore + Malaysia + Indonesia construction and manufacturing typically use grade 8.8 for structural, 10.9 for high-load, 12.9 for aerospace + automotive critical applications. Local stockists carry standard sizes M6-M36; specialty sizes import-ordered. Anti-seize lubricants for high-temperature applications (industrial plant) common; standard machinery oils for typical assembly. Tropical climate considerations: galvanic corrosion between dissimilar metals + bolt material is important in coastal Singapore + Malaysia.
10 Things to Know About Bolt Torque
T = K × F × d. Torque = nut factor × preload × diameter.
Nut factor K: 0.10 lubed, 0.20 dry, 0.30 plated. Captures all friction.
Standard preload target: 75% of proof load. Some apps 90%, some 50%.
ISO 8.8: 800 MPa tensile, 640 MPa yield. Most-used structural grade.
Torque accuracy: ±15-30% typical even with calibrated wrenches. Friction variation.
For critical apps: angle-controlled tightening or ultrasonic preload measurement.
Anti-seize on threads = lower K = lower torque for same preload. Don\'t mix specs.
Preload, not torque, is the goal. Torque is just a proxy.
Cyclic loading: preload relaxes 10-20% in first 1000 cycles. Re-torque sometimes needed.
Manufacturer specs override generic calc for safety-critical applications.
Frequently asked questions
Dimensionless coefficient bundling all friction effects — thread friction + head/nut bearing friction. K accounts for ~85% of input torque being converted to friction loss, with only ~15% becoming preload. Different lubricants and surface conditions change K significantly.
Friction varies bolt-to-bolt by 15-30% even within nominally identical conditions. Slight differences in surface roughness, lubricant distribution, thread quality cause it. This is why aircraft + nuclear + critical bolts use angle control or direct preload measurement.
75% is the most common default for static joints. Higher (85-90%): tension-controlled aerospace joints, prestressed concrete. Lower (50-60%): vibration-sensitive joints where preload relaxation is acceptable. Critical joints have manufacturer-specified preload — use that, not generic.
Two-step: (1) snug torque to remove slack + seat the joint. (2) Rotate by a specified angle (e.g. 90° or 180°). Since bolt elongation is proportional to angle past snug, this directly controls preload — independent of friction. Automotive engine head bolts often use angle control.
Same fundamental T = K·F·d formula. Inch (SAE) bolts use different grade designations (SAE J429: Grade 5, 8). Property class 8.8 ≈ SAE Grade 5; 10.9 ≈ SAE Grade 8. This calculator uses ISO metric — convert sizes (1/2 inch ≈ M12) + grades as needed.
Within first 100 hours of operation for some applications (engine bolts, freshly-installed flanges) due to "settling" + creep. After overhauls. After known thermal/shock events. Check manufacturer schedules. For static long-life joints (building structures), typically no re-torque needed under normal conditions.
No. All inputs stay in your browser.
Quality click-type wrenches: ±4% of setting in middle of range, worse at extremes. Beam-type: ±2% but harder to read. Digital: ±2-3%. Calibration drift: 5-15%/year typical. Calibrate annually. Always use wrench at 20-80% of range, not extremes.
Thermal expansion differences between bolt + clamped parts can dramatically change preload. High-temperature joints (turbines, exhaust manifolds): preload at room temperature is much higher than at operating temperature. Engineering accounts for thermal cycling via initial preload margin + creep-resistant alloys.
Shigley\'s Mechanical Engineering Design, Ch. 8. Industrial Fasteners Institute (IFI) standards. ASME PCC-1 for bolted flange joints. NASA NSTS 08307 for aerospace preload practices. RCSC Specification for structural bolted connections.
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